A cube corner type retroreflector, for example, that shown and described in the Heenan et al U.S. Pat. No. 3,541,606, which incorporates two or three different retroreflective areas, each area being comprised of a group of discrete reflector elements or units, each group having members with similar respective optical axes which are disposed collectively at an angle which differs from the corresponding angle in each of the other groups, suffers from the disadvantage that the total retroreflective surface region thereof has necessarily heretofore been comprised of such groups, and the total area each group occupies comprises a relatively large percentage of the total retroreflective surface region of such a given cube corner type reflector. Thus, if perchance a portion of the surface area of an individual retroreflector utilizing two or three such different groups of reflector elements therein is partially covered over as by a foreign body, so that, for example, the surface area occupied by one group of reflector elements is rendered non-functional, that reflector body itself is no longer fully retroreflective of light incident thereagainst, and thus that retroreflector is not retroreflective at the angles and to the extent previously served by the uncovered groups of reflector elements. This result, as a practical matter, can be regarded as having serious safety consequences, particularly in the area of reflectorized vehicles, such as bicycles, which are equipped with reflectors having multiple groups of discrete reflector elements. For example, a bicycle equipped with a reflector having two or three different groups of cube corner reflector elements therein, as indicated above, may no longer be seen by, for example, a motorist approaching such so-equipped moving bicycle at night from an angle of from about 40.degree. to 70.degree., for example, if such reflector's wide angle groups are obscured by a spatter of road mud, or the like. Consequently, in the cube corner reflector art, there is a need for a cube corner type retroreflector having two or three different groups of discrete cube corner reflector elements therein comprising the entire retroreflective region with each group having its members with similar respective optical axes which are disposed at different angles as taught in the prior art, but wherein the individual members of these different groups are so-distributed and so intermixed across the entire such retroreflective region of such reflector that a partial obscuring of that reflector's retroreflective region does not stop completely the generation of a desired, designed pattern of light retroreflection intended to be achievable with such reflector.
Because cube corner type retroreflectors comprised of molded transparent solid material have heretofore characteristically been manufactured from molds having incorporated thereinto, as the molding surface for forming cube corner retroreflective units, monolithic electroforms made from entire groups or clumps of faceted pin bundles wherein individual pins are appropriately faceted and arranged so as to produce an electroformable surface incorporating a plurality of discrete reflector units, it has heretofore not been possible to produce reflectors of the class indicated above wherein two or more different groups of cube corner reflector elements are disposed over the entire region of such reflector. Thus, as those skilled in the art of cube corner reflector manufacture well know, molds for cube corner retroreflective surfaces are prepared by a manufacturing sequence in which tiny pins, which commonly can be hexagonally shaped, having like facets formed at a forward end of each pin, are grouped into a pattern or bundle. The faceted pin ends of the bundle then serve as a form or surface upon which an electroform mold is made. Electroform molds are currently made by electroplating nickel or the like onto and over a pin bundle so that, in such process, all points, including the high points and the low points, respectively, over such a group of pins are reversed in exact mirror image fashion in the product electroform over their respective locations in the pin bundle. Then, using the product electroform, a mold is made in which transparent plastic reflectors are moldable. Because of the small size of the individual cube corner retroreflective units in such an electroform, and also because of cube corner retroreflective unit geometries, it has heretofore been necessary in the manufacture of molds for making cube corner retroreflectors to employ individual electroform structures wherein all of the discrete cube corner retroreflective units therein comprising a region of retroreflective faceted units have optical axes disposed substantially parallel to one another. Then, in the process of making a completed mold, having two or three different groups of retroreflecting areas, several different types of separately formed electroform structures are mounted together usually and typically in adjoining, adjacent relationship, each individual such electroform structure being comprised of a plurality of cube corner retroreflective units wherein the optical axes are respectively disposed parallel to one another, thereby to achieve a reflector of the character as described, for example, in the afore mentioned Heenan et al U.S. Patent.
In addition to the prior art constraint above discussed whereby cube corner retroreflectors were required because of electroform considerations to have distinct, separate regions comprised of groups of like cube corner reflector units, another serious problem has arisen particularly in the manufacture of cube corner-type retroreflectors employing angled cube corner retroreflective units with optical axes angled or inclined with respect to a region axis normal to the region wherein cube corner retroreflective units are arranged (the arrangement usually being planar in configuration). This problem concerns the fact that, in a mold being used to make a cube corner retroreflector comprised of transparent solid material (comprising, for example, an organic plastic, such as an acrylic resin, a polycarbonate resin, or the like), elevated temperatures and pressures are employed. As hot, fluid plastic is injected into the mold, air must rapidly and easily escape therefrom so that such plastic can promptly fill completely the mold cavity. Experience has now shown that in a prior art reflector mold assembly under use conditions, air apparently vents more readily from so-called standard cube corner retroreflective regions formed in an electroform structure (e.g. regions wherein the individual reflective unit optical axes are perpendicular to the region axis) than from so-called angled cube corner retroreflective regions formed in an electroform structure (e.g. regions wherein the optical axes of the individual reflective units are inclined to the region axis).
The differences in air venting capability are shown by the respective different mold cycle lives associated with molds employing electroforms for standard cube corner optics as opposed to those employing wide angle cube corner optics. Thus, for example, electroforms of standard type cube corner reflector units permit contemporarily typically perhaps one million impressions under commonly used molds and molding conditions for acrylic and polycarbonate resins, whereas electroforms of wide angle type cube corner reflector units permit contemporarily typically only perhaps about one hundred thousand impressions to be made with comparable molds and under comparable molding conditions with the same resins. This difference is believed to be caused by the fact that the wide angle electroforms tend to experience burning during a reflector molding operation, the burning being caused by the development of extremely high, localized pressures in the vicinity of the individual cavities defining portions of the cube corner retroreflective units in an electroform. For example, localized fluidic (hydraulic) pressures of typically perhaps 8,000 to 20,000 pounds per square inch, depending upon equipment and processing conditions, are achieved in the region of such a unit in a single molding cycle while concurrently localized elevated temperatures under molding conditions can exist for short periods. Such conditions are sufficient to oxidize and destroy surface portions of an electroform over a relatively short period of successive cycles in mold operation and use. Because individual electroform bodies are commonly very expensive to manufacture, cube corner reflector manufacturers desire to obtain as many cycles as possible from an individual mold employing electroforms in order to hold down the manufacturing cost of the individual product reflectors made therewith.
There has thus developed a real need in the art of making cube corner retroreflectors for electroform constructions or equivalent which permit long life for those portions of the mold containing incorporated thereinto cube corner retroreflective units of so-called wide angle (e.g., unit optical axes being inclined up to about 30.degree. relative to the region axis) retroreflective capability. The wide angle electroform constructions should have a mold cycle life at least approximately equal to that associated with so-called standard cube corner electroform constructions, and, furthermore, both types of electroform constructions should have long cycle lives in terms of capability of producing a product reflector having cheap manufacturing cost factors associated therewith for wide utilization in the marketplace by consumers. So far as is now known, no one has heretofore solved either the problem of air escape from molds incorporating wide angle cube corner reflective units defined in electroform bodies, or the problem of large area requirements for each group of cube corner retroreflective elements.